1. Field of the Invention
The invention is directed to an x-ray examination system having an x-ray source, particularly for computed tomography and for mammography.
2. Description of the Prior Art
In computed tomography, the physician would like to have the exposure plane, i.e. the plane of the body layer to be exposed, and thus the focal spot of the x-ray source, as close as possible to the actuation side of the gantry, i.e., that side of the gantry at which the physician is located for preparation for the exposure and possibly during the implementation of an examination. The actuation side of the gantry is usually provided with control and/or display elements.
Similarly, there is also a desire in mammography to bring the focal spot of the x-ray source as close as possible to a plane containing the sternum of a patient to be examined, and thus over the free end--representing the exposure side of the x-ray examination system--of a support provided for the breast of the patient, in order to be able to irradiate the tissue to be examined as completely as possible with the x-rays emanating from the focal spot, i.e. including the base of the breast, i.e. that region of the breast that forms the transition to the rib cage.
Since the x-rays emanating from the focal spot of the x-ray source leave the x-ray source through its radiation exit window, the above desires mean that the beam exit window of the x-ray source should be located as close as possible to the operating or exposure side of the x-ray examination system. A beam exit window that is located close to the end face of the x-ray source is in fact possible given x-ray tubes with a stationary anode (standing anode) operated as a single-pole tube, however, x-ray sources in computed tomography as well as in mammography usually contain rotating anode x-ray tube.
Due to the necessity, extremely high tube voltage, an x-ray tube utilized in x-ray sources provided for computed tomography is conventionally a two-pole x-ray tube and thus has an insulator at the anode side as well as at the cathode side. The length of this insulator determines the minimum spacing of the beam exit window from the corresponding end of the x-ray source. For this reason, x-ray tubes utilized in computer tomography are desired, to the extent possible, to reduce the overall structural length of the x-ray source, including insulators.
There are specific rotating anode x-ray tubes for x-ray sources provided for mammography that are operated as single-pole tubes, the drive being located at that side of the anode facing away from the patient. The spacing of the beam exit window from the end of the x-ray source at the cathode side is determined in such arrangements by the focal head length and the spacing between the cathode and the anode of the rotating anode x-ray tubes as well as by the spacing between the cathode-side end of the rotating anode x-ray tube and the cathode-side end of the x-ray source.
German OS 196 31 899 (corresponding to U.S. Pat. No. 5,883,936), German OS 41 08 591 and U.S. Pat. No. 4,788,705 disclose rotating bulb tubes, however, nothing can be derived from these publications about the installation position of the disclosed rotating bulb tubes in an x-ray examination system.
In mammography as well as in computed tomography, there is thus a need for an x-ray source having a beam exit window located extremely close to the actuation side or the exposure side.